Here's a win-win for all those bath lovers who struggle with poor sleep: New research suggests a soak in the tub before bedtime may shorten the time it takes to fall asleep.
A well-timed warm bath, or even a warm shower, also appears to prolong how long someone stays asleep, investigators found. And indications are that overall sleep quality improves as well.
Why? In large part, it has to do with lowering a person's body temperature.
Body temperature "starts to naturally decline as part of its natural [24-hour] cycle about one to two hours before the usual time of going to sleep," explained study author Shahab Haghayegh.
And a warm bath or shower can give that process a shove in the right direction, he explained, by boosting blood circulation from the inner body to the outer body. The result is a "very efficient removal of heat from the body, which causes a decline in body temperature," he said.
The trick is to both time and heat that bath to perfection.
"Yes, the temperature matters," stressed Haghayegh, a doctoral candidate in sleep research and bio-med engineering at the University of Texas at Austin.
"It should be warm. Not too hot or cold," he noted. "Actually, a too cold or too hot bath can have an effect opposite than that desired, causing an increase, rather than a decrease, in core body temperature, and disturbed sleep."
Timing is also important. "The optimal timing of bathing for cooling down of core body temperature in order to improve sleep quality and help with falling asleep faster is approximately one to two hours prior to going to bed," he said. Taking it outside that window can actually disrupt the natural body temperature cycle, he warned, and not in a good way.
But after analysing the findings of 17 previous investigations, Haghayegh and his colleagues found that a properly heated bath or shower taken at the right time for as little as 10 minutes can have a positive impact on sleep.
The review was published in the August issue of Sleep Medicine Reviews.
The studies in the review included all sorts of participants, including young, healthy soccer players, middle-aged patients struggling with traumatic brain injury, and older patients diagnosed with sleep apnea. Some even focused on cancer patients and those coping with heart disease.
But regardless of the type of person at hand, the review indicated that those who took a timely warm bath or shower effectively set in motion a process known as "water-based passive body heating".
And doing so reduced the time it took to fall asleep, also called "sleep onset latency."
The total time patients were able to spend asleep also went up. And warm baths appeared to serve as a booster of "sleep efficiency," meaning the amount of time a person spent in bed sleeping, relative to the amount of time spent in bed trying to sleep.
Sleep researcher Adam Krause, who was not involved in the study, said the sleep-promoting power of a warm bath or shower "has long been believed. And it's nice to see the literature provide support for it."
Krause is a doctoral candidate in psychology with the Center for Human Sleep Science at the University of California, Berkeley.
It may seem a bit counterintuitive, he acknowledged, given that it essentially involves exposing the skin to a certain amount of heat to trigger a drop in body temperature.
"[But] the net effect of this is a cooling of the core body and brain temperature, which is the necessary sleep-initiating cue the brain is waiting for," Krause explained.
"I think this is such a nice, simple and subtle technique to help with sleep," he added. "And it's always one of my main recommendations for people having trouble initiating sleep."
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